1. Field of the Invention
The invention relates to a current mirror circuit among electronic circuits which are used in various electronic apparatuses.
2. Related Background Art
A conventional current mirror circuit is constructed as shown in FIGS. 1 and 2.
The current mirror circuit of FIG. 1 has a circuit construction such that a constant current source 4 is connected to the collector side of a PNP transistor 2 in which the portion between the base and collector is short-circuited and a connecting point of the collector and base terminals is connected to a base terminal of another PNP transistor 6. Reference numeral 1 denotes a power source line. A collector current I.sub.out of the transistor 6 is generally expressed as follows by using a collector current I.sub.in of the transistor 2 ##EQU1## or is expressed as follows in consideration of the Early effect ##EQU2## where, h.sub.FE : current amplification factor
V.sub.CB : voltage between collector and base PA1 V.sub.A : early voltage
As will be obviously understood from the equation (1), however, I.sub.out depends on the magnitude of h.sub.FE. For instance, when h.sub.FE =30, I.sub.out =0.9375I.sub.in and an error of 6% or more occurs. From the equation (2), even when h.sub.FE =.infin., for instance, if V.sub.A =15 V and V.sub.CB =2 V, I.sub.out =0.88I.sub.in, so that there is a problem in that an error of 10% or more really occurs.
FIG. 2 is a diagram showing a current mirror circuit to reduce the dependency on h.sub.FE in the above two problems. An emitter of a transistor 3 whose collector is connected to a reference potential V.sub.Ref is connected to a base of the PNP transistor 2. A collector of the transistor 2 is connected to a base of the transistor 3. The rest of the construction is similar to that of FIG. 1. In the case of the circuit of FIG. 2, the collector current I.sub.out of the transistor 6 is generally given by ##EQU3## For instance, in a manner similar to the circuit of FIG. 1, when h.sub.FE =30, I.sub.out =0.998I.sub.in and a mirror coefficient has a value which is near 100%. However, the dependency on the voltage between collector and base due to the Early effect still remains and there is a problem in that a large error occurs in a manner similar to the circuit of FIG. 1.